The electrophoretic effect is well known and the prior art is replete with a number of patents and articles which describe the effect. As will be recognized by a person skilled in the art, the electrophoretic effect operates on the principle that certain particles, when suspended in a medium, can be electrically charged and thereby to migrate through the medium to an electrode of opposite charge. Electrostatic printing and electrophoretic image displays (EPID) utilize the electrophoretic effect to produce desired images.
In prior art EPIDs, colored dielectric particles are suspended in a fluid medium of an optically contrasting color as compared to the dielectric particles. The colored electrophoretic particles are then selectively caused to migrate to, and impinge upon, a transparent screen, thereby displacing the fluid medium from the screen surface and creating the desired image. EPIDs commonly use readily manufactured light colored electrophoretic particles suspended in media which contains dark color dyes. Such EPIDs are exemplified in U.S. Pat. Nos: 4,655,897 to DiSanto et al., 4,093,534 to Carter et al., 4,298,448 to Muller et al., and 4,285,801 to Chaing. The dark medium can not be completely displaced by the suspended particles because there is always some dark liquid left on the screen or around the particle surface. The resulting images therefore suffer some loss of contrast.
An alternative way of creating contrasting images is to have a dispersion that contains both black and white dielectric particles having opposite charges. When an electric field is applied, the black and white particles are caused to migrate in opposite directions due to their opposite polarity. This creates the desired image with black and white contrast. Since the black and white dielectric particles have opposite charges, they will completely separate and migrate in opposite directions under the applied electric field, without leaving any black particles on the white image or any white particles on the black image. This produces highly contrasted black and white images. Ideally, the black and white dielectric particles must have optimum electrophoretic mobilities and be thermodynamically stable in the same medium in order to have reasonable imaging speed and to prevent flocculation due to the electrostatic attraction between the oppositely-charged particles.
In nature, as well as in industry, aggregation of oppositely-charged particles in dispersion has been shown in a large variety of fields such as soil conditioning, water purification, filtration and adhesion or aggregation of biological cells. Good examples are given in Colloids and Surface, 6, 83(1983); 6, 101(1983) and in Journal of Chemical Society Faraday, 76, 665(1980), in which the particles adsorption isotherms of small positively-charged polystyrene particles onto large negatively-charged polystyrene particles have been reported. In the prior art, it has proven difficult to produce an electrophoretic dispersion containing oppositely-charged particles without flocculation, particularly in non-aqueous systems. Theoretically, with the addition of a proper polymeric stabilizer, it is possible to stabilize two oppositely-charged particles in the same medium if the steric repulsion provided from the absorbed polymer layer can overcome the electrostatic attraction between the two oppositely-charged particles. However, very few surfactants have been found to be able to produce enough steric repulsion to stabilize oppositely-charged particles without trading off their electrophoretic mobility. Accordingly, there is no known electrophoretic image display, which gives a highly contrasted black and white image.